Building-out unit



Nov. 7, 1939. v a. SLADE BUILDING-OUT UNIT Filed Oct. 22, 1938 11vVENTOR B. $1.14 DE Bl ,4 TTOIRNEY LOADING SECTION N mpm E THM A 8 mew. u0 R w m L I w um m 0 T HE C C L x m 4W6 .l

E15 C IN 5 ULA TED CA BLE OUT UNIT Patented Nov. 7, 1939 UNITED STATESPATENT OFFICE Bell Telephone Laboratories,

Incorporated,

New York, N. Y., a corporation of New York Application October 22, 1938,Serial No. 236,506

9 Claims.

This invention relates to artificial lines and more particularly todevices which simulate the electrical characteristics of portions oftransmission lines.

As is well known in the art, cable transmission lines are usually loadedat definite points with inductors to neutralize the distributedcapacitance of the line. The length of the line between loading pointsis called a loading section and the inductors or loading coils aremanufactured to have sufficient inductance to satisfy the requirementsof a full loading section or a definite partial section. When repeatersor other transmission apparatus are inserted in the lines practicalconsiderations are such that generally such apparatus is inserted atpoints other than those which define a full loading section or adefinite partial section. It then becomes necessary to insert in theline apparatus which will simulate the characteristics of a length ofthe line sufficient to make the impedance of the line from the lastloading point to the point of insertion of the transmission apparatusthe equivalent of a loading section or a definite partial section.Apparatus for thus simulating the impedance of a part of a section iscalled a building-out unit. With ordinary cables it is, in general,merely necessary to insert capacitors of the proper value to build outthe line to the corresponding value of the full or partial loadingsection.

Under certain conditions it is found economically desirable to applycarrier currents to open wire lines. However, when it becomes necessaryto run carrier transmission lines below ground as in cities or underwater, open wire lines cannot be used and it has been found advantageousto use disc insulated cable of the general type disclosed in Fig. 3 ofU. S. Patent 2,086,629 to S. P. Mead, July 13, 1937, but having anadditional lead sheath.

Disc insulated cable has a large distributed inductive reactance as wellas distributed capacitive reactance. Building-out units for discinsulated cable must, therefore, have both inductive and capacitivereactances to simulate the characteristics of the disc insulated cable.

An object of this invention, therefore, is a building-out unit havingboth inductive and capacitive reactances in the same ratio as those ofthe line which is to be built out.

A further object of the invention is a buildingout unit which may bealtered in the field to give the required specific values of inductanceand capacitance and maintain the ratio constant.

These objects are attained in one embodiment of the invention by using asolenoidal type of Winding on a long thin core, the winding being suchthat when turns thereof are removed the ratio of inductance tocapacitance remains substantially constant from full length down toabout one-third of its original length.

This invention will be better understood and further objects andfeatures will be apparent from the following description and attacheddrawing forming a part thereof and in which Fig. 1 is a perspective viewof a building-out unit with part of the enclosing case removed to showthe coil;

Fig. 2 shows a modified unit also with a part of the enclosing caseremoved;

Fig. 3 shows a specific method of winding;

Fig. 3A is a schematic of the winding of Fig. 3;

Figs. 4 and 5 show modifications of the method of winding;

Figs. 4A and 5A show the respective schematics; and

Fig. 6 is a schematic diagram of a transmission circuit illustrating theuse of a building-out unit.

Referring now particularly to Fig. 1 a metallic case i having mountinglugs 2 soldered or welded thereto encloses a long thin single-layer coil3 comprising two windings wound as a parallel pair I I, l2, one for eachside of a transmission line, on an insulating material core 4, andhaving insulating material spool heads 5. The coil is held in position,that is, spaced from the sides of the case by a piece of insulatingmaterial 6 which forms a terminal plate. A pin 18 or projection formedin the bottom of the case enters a recess in the bottom spool head tofix the coil in the lower end of the case. The ends of the windings arebrought out to terminals l which are staked or otherwise fastened toterminal plate 6.

Terminal plate 6 is heldin position by the cover 8 by means of longrivets 9. The space between plate 5 and the cover 8 is filled with aninsulating compound or insulating block Iii. The winding is given a waxcoating and the cover 8 is preferably soldered to the casing i.

Fig. 2 illustrates a modified type of unit in which the absolute valuesof inductance and capacitance as well as the ratio of capacitance toinductance are different from those of the coil of Fig. 1. In this unitthe core 4 is substantially rectangular in cross-section and the winding3 comprises a parallel pair II, 12'. The enclosing case I is of the sameshape and size as that of Fig. 1 so that a single type of case will besatisfactory for either type of coil.

To facilitate removal of turns, no spool heads per se are used. To holdthe coil in position in the case a thin rectangular piece of insulatingmaterial I9 is fitted into a slot in the upper end of the core. Piece I9is of such dimensions that it touches opposite sides of the case and isjust flush with the terminal plate 6. A cylindrical rod M which istightly fitted into holes through the core 4' and piece I9 spaces thecoil from the other sides of the case.

On the bottom there is a spacing piece l3 and a hole in the end of thecore into which pin is fits to fix the lower end of the coil in thecase.

The case is provided with mounting lugs 2' and the structure of the topof the case is the same as that of Fig. 1.

In terminating a length of disc insulated cable at points intermediateto the loading points, the cable can be built out to a certain extent bymeans of building-out units of fixed values. It so happens, however,that certain adjustments must be made in the field. The structures ofFig. 1 and Fig. 2 lend themselves readily to field adjustments since thecover and coil of either unit may be easily removed from the case. Whenthe coil is removed turns of the windings may be removed to provide thecorrect values of inductance and capacitance.

With a long, thin coil such as that shown in Figs. 1 and 2 theinductance varies substantially linearly from full length down to aboutonethird the original length. Since the capacitance of the coil is thesum of the individual capacitances existing between the individual turns'61: the windings, it varies linearly also. Hence by removing turns fromthe core the inductance and capacitance are reduced while the ratio ofcapacitance to inductance remains constant.

Fig. 3 illustrates more clearly the windings when composed of a parallelpair. One winding comprises conductor ll corresponding to H of ,Fig. 1and H of Fig. 2, while the second winding comprises conductor [2'corresponding to 52 of Fig. 1 and I2 of Fig. 2. To have these windingsconnected in series aiding around the loop, some such expedient must beused as that shown in Fig. 3A. In this case conductor i2 is shown asdoubled back on itself. The capacitances that exist in this coil to givethe proper building-out capacitance are shown dotted at l5.

Fig. 4 shows a method of winding in which there need be no doubling backof one conductor to provide a series-aiding connection to the paircomprising a transmission line. In this case, for example, conductor i!is given one turn in one direction around the core and then conductor!2' is given one turn in the opposite direction around the core.Conductor ii is then given a second turn and then conductor i2 is givenits second turn. This is carried on until the desired number of turnshave been made. This method of winding, of course, requires that theconductors cross once per turn. This cross-over, however, will have nodetrimental effect. The capacitances between the turns as shown in Fig.4A are substantially the same as in Fig. 3A.

Fig. 5 shows an arrangement where less capacitance is needed to give thedesired ratio. In this case conductor I l is given two turns around thecore in one direction and then conductor i2 is given two turns aroundthe core in the opposite direction. Conductor H is then given two moreturns, followed by two turns of conductor i2 until the desired number ofturns has been applied. In this case there is a cross-over of two turns,as shown.

Fig. 5A shows the distribution of the capacitances, and it is to benoted that these capacitances l6 exist only between adjacent turns ofthe individual conductors and since there are two turns of one conductorfollowed by two turns of the second conductor the total capacitance is.reduced. It is, of course, understood that there exist other extremelyminute capacitances between various parts of the winding but these areso small as to have no appreciable effect. As shown, there exist threeeifective capacitances it with four turns of each conductor wound as inFig. 5, while with four turns of each conductor wound as in Figs. 3 and4 there exist seven capacitances.

To give a further decrease of capacitance one conductor may be giventhree turns and then the second conductor may be given three turns, orto further decrease the capacitance the number of turns of eachconductor may be four, five, etc.,

depending upon the capacitance desired or the ratio of capacitance toinductance required.

Figs. 1 and 2 may be considered from a design standpoint as limitingcases as to the shape of the building-out unit. units under discussionis broadly proportional to the length of the adjacent wires or to theperimeter of the cross-section perpendicular to the axis or the unit. Inthe same way the inductance of the unit is broadly proportional to thearea of 3.:

the cross-section of the unit. I-Ience with a cylindrical unit in whichthe perimeter of the cross-section is a minimum for a given area, thecapacitance is a minimum for the inductance.

Now at the other extreme consider a very thin rectangle. In this casethe perimeter is large as compared to the area and it is also found thatthe capacitance is large as compared to the inductance when the unit isconnected to be series aiding in the loop.

Therefore, in arriving at the proper shape for the unit, disregardingmethods of windings, as, for example, those of Figs. 4 and 5, thedesired ratio of capacitance to inductance will be found with some shapevarying in cross-section from .the circle of Fig. 1 to the rectangle ofFig. 2. Because of practical winding conditions the corners of therectangular form must be somewhat rounded.

As specific examples of coils according to this invention, a cylindricalcoil having a ratio of length to diameter of about 5 or 6 with thelength approximately 2 inches and eighty turns had a normal inductanceof 11.7 microhenries and a capacitance of 200 micromicro- "=7 farads. Acoil of oval or substantially rectangular cross-section had a length ofapproximately 3 inches and a thickness of approximately inch, giving alength-to-thickness ratio of approximately 8% or 9. With one hundredturns, this coil had a maximum inductance of 23.4 microhenries and acapacitance of 400 micromicrofarads, the ratio of capacitance inmicromicrofarads to inductance in microhenries being about 17 in eachcoil. Coils having a normal ratio of 17 are considered satisfactory ifthey have a ratio from 16 to 18.

These coils are designed for use with a fourwire disc insulated cable inwhich the loop inductance for one mile of a diagonally opposite pair is1,445 micrchenries and the corresponding capacitance is 24,670micromicrofarads at a frequency of kilo-cycles, the ratio of capacitanceto inductance being approximately 17.

It is to be understood in the foregoing discus- The capacitance of thesion involving the number of turns of the coil that a coil wound with aparallel pair and spoken of as one of one hundred turns has fifty turnsof the parallel pair. As to the removal of turns, the removal of oneturn means the removal of one turn of the pair. For example, a coilhaving one hundred single turns and having one turn of the pair removedwould have a capacitance and an inductance one-fiftieth less than theoriginal coil. This is also true of coils having the crossover type ofwinding illustrated in Figs. 4 and 5. A coil having one hundred singleturns of the cross-over type has fifty turns in each direction and theremoval of one turn means the removal of one turn of each winding. Theremay be cases in practice where only one turn of one winding is removed.In such cases the resultant unbalance is of less importance than thespecific value of capacitance and inductance desired.

Fig. 6 is a schematic circuit diagram showing loading coils 23 and 24inserted in a transmission line comprising a disc insulated cable 25. Abuilding-out unit 26 is shown as included in station 2'! which housesrepeaters or other terminating apparatus 28. The distance betweenloading coils 23 and 24 is indicated and defined as a full loadingsection while the distance between loading coil I24 and station 27 orthe building-out unit 26 in station 2? is indicated as less than a fullloading section thereby making the building-out unit-necessary. Thisfigure illustrates that when a transmission line is to be terminated ata point other than a loading point, the length of the line which isphysically shorter than a loading section may be built out by means ofapparatus which will make such a section equivalent electrically, to afull section. Such an electrically builtout section can then be properlyterminated.

Certain specific forms of the invention have been described andillustrated as examples. The invention, however, is to be limited onlyby the scope of the appended claims.

What is claimed is:

. 1. A building-out unit for a transmission line having capacitive andinductive reactance which comprises a coil in which the ratio of totalinductance to total capacitance is substantially equal to that of saidline.

2. A building-out unit for a transmission line having capacitive andinductive rcactance which comprises a coil having a single-layer windingthe loop inductance and the capacitance between the Wires of thewindings of which are substantially equal to the loop inductance and thecapacitance between the wires of a predetermined portion of saidtransmission line.

3. A building-out unit for simulating the impedance of a section of aloaded transmission line which comprises a coil having a non-magneticcore and a single-layer winding on said core the inductance andcapacitance of which have the same ratio as that of the section of theline to be built out.

4. A building-out unit for a loaded transmission line which comprises acoil having a singlelayer winding the ratio of the inductance tocapacitance of which remains substantially constant for more than halfof the number of turns of said Winding.

5. Means for building out a section of a transmission line to make saidsection have an inductance and a capacitance equal to the inductance andcapacitance of another section of said line which comprises a core of alength several times its diameter, a single-layer winding on said core,said winding comprising a plurality of conductors wound on said core asuificient number of turns to give said winding an inductance and acapacitance equal to the difference between the inductance andcapacitance of the section to be built out and the inductance andcapacitance of said other section.

6. A building-out unit for a loaded transmission line which comprises acoil having a ratio of length to thickness which may vary from 5 to 9and having a ratio of capacitance as measured in micrcrnicroiarads toinductance as measured in microhenries of approximately H and in whichthe ratio of capacitance to inductance remains substantially constantfor approximately two-- thirds the length of the coil.

'3. In a transmission line loaded at specified points, the distancebetween such points defining a loading section, a termination of saidline at a point other than one of said specified points and means tosimulate the electrical characteristics of said line inserted at saidtermination to make the section of line from the preceding loading pointto the termination electrically equivalent to a full loading section,which means comprises a coil having a ratio of length to thickness ofapproximately 9 and a ratio of capacitance to inductance substantiallythe same as the ratio of capacitance to inductance of said line.

8. In a transmission line loaded at specified points, the distancebetween such points do ning a loading section and having both inductiveand capacitive reactance characteristics, a termination of said line ata point other than one of said loading points and means inserted in saidline to build out said incomplete section to the electrical equivalentof a full section which comprises a coil having a cross-section otherthan circular and having capacitive and inductive reactances which varysubstantially linearly for two-thirds oi the length of said coil.

9. A building-out unit for a transmission line comprising a cylindricalcore having a length of the order of five to nine times the diameter, a

single layer winding on said core, said winding comprising a pluralityof wires, the ratio of the capacitance in micromicrofarads between thewires to the loop inductance in inicrohcnries of the winding beingsubstantially the same as that of the transmission line which the unitis to build out, said ratio remaining substantially constant for morethan half the length of said windin BENJAMIN SLADE.

